Defects in the heat treatment of moulds and their prevention

1 mold surface has soft spots

The surface of the mold after heat treatment has soft spots, which will affect the wear resistance of the mold and reduce the service life of the mold.

(1) Cause

1) The surface of the mold has scale, rust, and local decarburization before heat treatment.

2) After the mold is quenched and heated, the quenching quenching medium is selected improperly, and impurities in the quenching medium are excessive or aged.

(2) Preventive measures

1) The scale and rust must be removed before the heat treatment of the mold. The surface of the mold should be properly protected during quenching and heating. The vacuum furnace, salt bath furnace, and protective atmosphere furnace should be used as much as possible.

2) When the mold is cooled by quenching and heating, an appropriate cooling medium should be selected. The cooling medium used for a long period of time must be filtered or replaced regularly. 2. Poor organization before heat treatment

The final spheroidizing structure of the mold is uneven and the spheroidization is not perfect. The organization has mesh, ribbon and chain carbides, which will make the mold easy to crack after quenching and cause the mold to be scrapped.

(1) Cause

1) Severe carbide segregation exists in the original microstructure of the die steel material.

2) Poor forging process, such as high forging heating temperature, small deformation, high stopping temperature, slow cooling speed after forging, etc., to make the forging structure coarse and there are mesh, belt and chain carbides, so that the ball It is difficult to eliminate the annealing.

3) Poor spheroidizing annealing process, such as annealing temperature is too high or too low, isothermal annealing time is short, etc., can cause uneven spheroidization annealing or spheroidization.

(2) Preventive measures

1) Generally, mold steel materials with good quality should be selected as far as possible according to the working conditions of the mold, the production volume, and the toughening properties of the material itself.

2) Improve the forging process or use a normalizing pre-heat treatment to eliminate the inhomogeneities of network and chain carbides and carbides in raw materials.

3) Solid carbide refinement heat treatment may be performed on high-carbon mold steel with severe segregation of carbides that cannot be forged.

4) Formulation of the correct spheroidizing annealing process specification for the forged die blanks can be performed by quenching and tempering heat treatment and rapid spheronization annealing.

5) Reasonably install the furnace to ensure the uniformity of the mold blank temperature in the furnace. 3. The mold produces quench cracks

The cracking of the mold after quenching is the biggest defect in the mold heat treatment process, which will scrap the processed mold and cause great loss in production and economy.

(1) Cause

1) Severe mesh carbide segregation exists in the mold material.

2) There is a machining or cold deformation stress in the mold.

3) Improper heat treatment of the mold (too fast heating or cooling, improper selection of quenching cooling medium, low cooling temperature, long cooling time, etc.).

4) The shape of the mold is complex, uneven thickness, with sharp corners and threaded holes, so that thermal stress and tissue stress are too large.

5) The mold quenching heating temperature is too high to cause overheating or overheating.

6) The quenching of the mold is not timely or the tempering holding time is insufficient.

7) Mold Rework When quenched and heated, quenching is again performed without intermediate annealing.

8) Mold heat treatment, improper grinding process.

9) After EDM of the mold heat treatment, there are high tensile stress and micro-cracks in the hardened layer. (2) Preventive measures

1) Strictly control the intrinsic quality of mold raw materials

2) Improve the forging and spheroidizing annealing process, eliminate mesh, ribbon, chain carbides and improve the uniformity of the spheroidized structure. ,

3) After the mechanical processing or after the cold deformation of the mold should be stress relief annealing (> 600 °C) and then heat quenching.

4) For molds with complex shapes, plugs of asbestos plugs should be used to block dangerous cross-sections and thin-walled areas, and to use graded quenching or isothermal quenching.

5) Annealing or high temperature tempering is required for rework or refurbishment of molds.

6) The mold should be preheated during quenching and heating, precooling measures should be taken when cooling, and the appropriate quenching medium should be selected.

7) The quenching heating temperature and time should be strictly controlled to prevent the mold from overheating and overheating.

8) The mold should be tempered immediately after quenching, the holding time should be sufficient, and the high-alloy complex mold should be tempered 2-3 times.

9) Choose the right grinding process and the right grinding wheel.

10) Improve the die EDM process and perform stress relief and tempering. 4. The microstructure of the mold after quenching

The coarse structure of the die after quenching will seriously affect the mechanical properties of the die, which will cause the die to fracture and seriously affect the service life of the die.

(1) Cause

1) The mould steel is confused. The actual quenching temperature of the steel is much lower than the quenching temperature of the required mould material (such as 3Cr2W8V steel for GCr15 steel).

2) Before the quenching of the die steel, the correct spheroidization process was not performed, and the spheroidization structure was poor.

3) The mold quenching heating temperature is too high or the holding time is too long.

4) Improper placement of the mold in the furnace, prone to overheating near the electrode or heating element area.

5) For molds with large section changes, improper selection of quenching heating process parameters results in overheating at thin sections and sharp corners.

(2) Preventive measures

1) Steel products should be strictly inspected before they are put into storage, and steel materials must be strictly prevented from being mixed.

2) Correct forging and spheroidizing annealing should be performed before the mold is quenched to ensure good spheroidization.

3) Correctly formulate the mold quenching heating process specification, strictly control the quenching heating temperature and holding time.

4) Periodically detect and calibrate the temperature meter to ensure that the meter works properly.

5) The mold should be kept at a proper distance from the electrode or heating element when it is heated in the furnace.